In a conventional electric power system, energy supply has been conducted from a power plant such as an atomic power plant, a thermal power plant, a hydraulic power plant, etc, by means of a large-scale power grid. Meanwhile, in recent years, a concept called “microgrid” in which a small-sized distributed power supply (photovoltaic, wind turbine, biomass, etc.) is connected to constitute a power network for energy supply to a predetermined area has been proposed and is now becoming widespread. Such a microgrid-based energy supply system realized by the small-sized distributed power supply is required to perform connected operation that controls power generation based on system connection such that purchasing electric power from a commercial system becomes constant at normal operation time, while in the event of emergency such as power failure, to perform load following operation which is islanded operation that supplies high quality (fluctuation in voltage and frequency are small) power to a microgrid system.
In constructing the microgrid, how the amount of power supply, which varies momentarily, is balanced is the most important issue. As factors causing the power supply amount to vary, a variation in a load, a variation in the power generation of a small-sized distributed power supply such as wind turbine power generation or photovoltaic power generation (hereinafter, variations in the wind turbine power generation and photovoltaic power generation are collectively referred to as “electric power variation”), and the like can be exemplified.
The electric power variation has various frequency components from quite abrupt variation to comparatively gentle variation depending on the load or power generation state of the small-sized distributed power supply. By combining distributed power supplies having various load following characteristics, it is possible to suppress the variation of all the frequency components of the electric power variation. Specifically, storage facility such as a secondary battery or an electric storage facility is used to cope with a variation (quite abrupt variation) in high-frequency components and a power generation facility such as a gas engine is used to cope with a variation (comparatively gentle variation) in low-frequency components, whereby the electric power variation can be suppressed.
Further, there is known a system that realizes the load following operation by managing power demand of a building by linkage of power receiving/transforming facility of a commercial system and a distributed power supply (refer to, e.g., Patent Document 1) and a method that utilizes a storage facility when an original operation plan has been significantly modified, to realize stable system operation (refer to, e.g., Patent Document 2).    Patent Document 1: JP-A-2005-160286    Patent Document 2: JP-A-2007-215290